Method for producing a sheet or strip from an aluminium alloy, and a sheet, strip or molded part produced thereby

ABSTRACT

A method for producing a sheet or strip from an aluminum alloy and a sheet, strip or molded part produced thereby are disclosed. A rough surface and stretcher strain marks can be avoided if the cold-rolled sheet or strip with a particular composition and microstructure is subjected to a heat treatment with a recrystallization annealing and a subsequent accelerated cooling.

FIELD OF THE INVENTION

The invention relates to a method for producing a sheet or strip from an aluminum alloy and to a sheet, strip, or molded part produced thereby.

DESCRIPTION OF THE PRIOR ART

In order to adjust the strength and formability or ductility, more particularly deep-drawing formability in a 5xxx-aluminum alloy or aluminum alloy with an Al—Mg basis, it is known to provide the sheet or strip or more precisely the metal structure of the aluminum alloy sheet or strip with a finer average crystal grain size, namely of 60 μm or, according to EP0507411A1, of less than 50 μm. A finer crystal grain size of 60 μm or less of this kind disadvantageously involves the risk of the occurrence of type A stretcher strain marks, namely Lüders bands, on the surface of the plastically deformed sheet or strip. Al—Mg—Mn alloys thus have only a limited suitability, for example, for outer shell components in vehicle body construction, which require ssf quality (stretcher strain free) or what is also known by its German abbreviation ffa quality (ffa=fließfigurenarme [low stretcher strain]), i.e. a freedom from or reduction in type A stretcher strain marks.

SUMMARY OF THE INVENTION

The object of the invention, therefore, is to create a method for producing a sheet or strip from an aluminum alloy having Mg as one of the main alloying elements and to create a sheet or strip of the type described above that has a comparatively high strength and formability and is of ssf quality or ffa quality. The method should also be easy to use and reproducible.

According to the invention, the sheet or strip in the method is composed of an aluminum alloy, namely with the composition of from 2.0 to 5.5 wt % magnesium (Mg), from 0.2 to 1.2 wt % manganese (Mn), optionally up to 0.45 wt % silicon (Si), optionally up to 0.55 wt % iron (Fe), optionally up to 0.35 wt % chromium (Cr), optionally up to 0.2 wt % titanium (Ti), optionally up to 0.2 wt % silver (Ag), optionally up to 4.0 wt % zinc (Zn), optionally up to 0.8 wt % copper (Cu), optionally up to 0.8 wt % zirconium (Zr), optionally up to 0.3 wt % niobium (Nb), optionally up to 0.25 wt % tantalum (Ta), optionally up to 0.05 wt % vanadium (V), and the remainder comprised of aluminum and inevitable production-related impurities, with up to at most 0.05 wt % of each and all together totaling at most 0.15 wt %.

The method has the following method steps:

-   -   casting of a rolling slab,     -   hot rolling of the rolling slab into a hot-rolled sheet or         strip;     -   cold rolling of the hot-rolled sheet or strip to a final         thickness;     -   heat treatment of the sheet or strip that has been cold-rolled         to the final thickness, including recrystallization annealing         with subsequent accelerated cooling;

Optionally, the method can have the following method steps:

-   -   homogenization of the rolling slab;     -   intermediate annealing of the sheet or strip in the cold rolling         of the hot-rolled sheet or strip to a final thickness     -   stabilization of the sheet or strip, which has undergone         accelerated cooling, in the heat treatment;

According to the invention, before the heat treatment, the sheet or strip that has been cold-rolled to the final thickness has at least one, more particularly primary, intermetallic phase with first particles having an average particle size of 5 μm to 10 μm (measured using the ASTM E112 linear intercept method)—this by means of the method steps preceding the heat treatment. For example in that at least the casting and the cold rolling, more particularly after the intermediate annealing, are adjusted relative to each other in such a way that the sheet or strip has at least one intermetallic phase with first particles having an average particle size of 5 μm to 10 μm. These first and thus primary particles are relatively coarse. These particles of the primary phase also have a high stability—even relative to a subsequent recrystallization annealing or relative to a subsequent heat treatment.

With such a composition and microstructure, it is possible to produce a sheet or strip with a high strength and formability and of an ssf quality or ffa quality—namely if after the heat treatment, this sheet or strip that has been cold-rolled to the final thickness also has an average crystal grain size D of ≤60 μm (measured using the ASTM E112 linear intercept method) and the average crystal grain size D in mm and the number A of first particles per mm² in the aluminum alloy satisfy the condition √{square root over (D)}*A>1.8—for example in that the recrystallization annealing of the heat treatment is performed in such a way. Because of the different thermal expansion coefficients, an accelerated cooling following the recrystallization annealing causes internal stresses in the structure to occur, namely between the aluminum matrix and the first particles of the intermetallic phase, which ensures that there is a sufficient number of free dislocations at the first particles of the primary intermetallic phase. As a result, Lüders band dislocations are not necessarily or inevitably produced during the forming of the sheet or strip. This is also true in the event of unfavorable deformations or complex geometries in the formed sheet or strip.

This method is also easy to use and has an extremely high reproducibility, for example due to a water cooling for the accelerated cooling, for producing a sheet or strip in ssf quality or ffa quality.

The number of dislocations in the sheet or strip can be further increased in the method if √{square root over (D)}*A is >2. More particularly, if √{square root over (D)}*A is >2.5, then the sheet or strip can satisfy comparatively high quality requirements without having to also fear the occurrence of stretcher strain marks such as type A Lüders bands on the surface of the formed sheet or strip, even in the case of comparatively complex geometries or unfavorable plastic deformations.

The method can be further improved in terms of reproducibility if in the heat treatment, the recrystallization annealing takes place by means of holding at a temperature of 300° C. (degrees Celsius) or more, more particularly up to 600° C. This can improve even more if the recrystallization annealing takes place at 450° C. to 550° C. In addition, this annealing temperature can be enough to pre-stress the structure by means of an accelerated cooling sufficiently to produce the dislocations at the first particles, which subsequently make Lüders band dislocations unnecessary.

This is more particularly the case if the heated sheet is cooled in an accelerated manner at a cooling rate of at least 10 K/s (Kelvin per second), more particularly at least 20 K/s or at least 50 K/s, wherein this accelerated cooling can more particularly be carried out to below 180° C., more particularly to room temperature.

It is possible to ensure that first particles are embodied as large enough in the average particle size if the rolling slab is solidified by maintaining a cooling rate (or cooling speed) of <2.5° C./s. This can be further improved if the cooling rate is <2° C./s or <1° C./s or <0.75° C./s. In addition, this can counteract a possible reduction in the average particle size by means of subsequent method steps, for example by means of the cold rolling, in order to ensure an average particle size of 5 μm to 10 μm before the heat treatment. In addition, the optional homogenization can take place by means of holding at 450° C. to 550° C. for at least 0.5 h.

The hot rolling can take place at 280° C. to 550° C.

The cold rolling to the final thickness can be carried out with a degree of rolling reduction of from 10% to 65%, more particularly from 20% to 50%. More particularly, it can be advantageous if the cold rolling after the intermediate annealing is carried out with a degree of rolling reduction of from 10% to 65%, more particularly from 20% to 50%, in order to improve the reproducibility of the average particle size of 5 μm to 10 μm.

The optional intermediate annealing can take place by means of holding at 300° C. to 500° C.

The optional stabilization can take place by means of holding at 80° C. to 120° C. for at least 0.5 h.

An average particle size of 5 μm to 10 μm before the heat treatment can more particularly be assured if the product of the degree of rolling reduction in % after the intermediate annealing and the cooling rate in ° C./s satisfies the condition 10≤degree of rolling reduction*cooling rate≤50, more particularly 20≤degree of rolling reduction*cooling rate≤45.

If the intermetallic phase has an Al—Mn basis, then it is possible to produce the dislocations in the aluminum alloy that enable stretcher strain marks to be avoided in a particularly reliable way. Preferably, the intermetallic phase is of the Al₁₃(Mn,Fe)₆ type or of the Al₁₆FeMn₃Si₂ type or of the Al₁₂Mn type or of the Al₆Mn type. These first particles of the primary phase are a particularly stable phase. It is also conceivable for the primary phase to constitute the intermetallic phase in order, in combination with the heat treatment of the sheet or strip, to produce a sufficient number of dislocations.

The method can achieve high strength and formability while avoiding orange peel and stretcher strain marks if the aluminum alloy (with an Al—Mg—Mn basis) has from 4.0 to 5.0 wt % magnesium (Mg) and/or from 0.2 to 0.5 wt % manganese (Mn).

Particularly high strength can be achieved if the aluminum alloy also has from 2.0 to 4.0 wt % zinc (Zn) (Al—Mg—Zn basis). Optionally, this aluminum alloy can also have up to 0.8 wt % copper (Cu).

If the sheet or strip is composed of an aluminum alloy, namely with the alloy contents from 2.0 to 5.5 wt % magnesium (Mg), from 0.2 to 1.2 wt % manganese (Mn), optionally up to 0.45 wt % silicon (Si), optionally up to 0.55 wt % iron (Fe), optionally up to 0.35 wt % chromium (Cr), optionally up to 0.2 wt % titanium (Ti), optionally up to 0.2 wt % silver (Ag), optionally up to 4.0 wt % zinc (Zn), optionally up to 0.8 wt % copper (Cu), optionally up to 0.8 wt % zirconium (Zr), optionally up to 0.3 wt % niobium (Nb), optionally up to 0.25 wt % tantalum (Ta), and the remainder comprised of aluminum and inevitable production-related impurities, with up to at most 0.05 wt % of each and all together totaling at most 0.15 wt %, then this provides an alloy composition with which it is possible to achieve a sufficiently high strength and formability/ductility—of the kind that is required, for example, for outer shell components in vehicle body construction.

Freedom from orange peel and stretcher strain marks, among other things Lüders bands, in the formed sheet or strip can be achieved if this sheet or strip has an average crystal grain size D of ≤60 μm (measured using the ASTM E112 linear intercept method) and at least one, more particularly primary, intermetallic phase with first particles having an average particle size of 5 μm to 10 μm (measured using the ASTM E112 linear intercept method) and the average crystal grain size D in mm and the number A of first particles per mm² in the aluminum alloy satisfy the condition √{square root over (D)}*A>1.8. It is also necessary for the sheet or strip to have been subjected to a heat treatment, including recrystallization annealing with subsequent accelerated cooling and optionally a stabilization of the sheet or strip that has undergone accelerated cooling. As a result, dislocations are produced at the first particles in the structure of the sheet or strip. These first and thus primary particles are also stable relative to the heat treatment that is used to further adjust the microstructure of the sheet or strip.

Thus the average crystal grain size D of ≤60 μm according to the invention results in the fact that the comparatively fine crystal grain of the sheet or strip enables achievement of a high strength and formability.

The latter, however, is not impaired by the presence of stretcher strain marks on the surface of the formed sheet or strip since according to the invention, the first particles that are present in the sheet or strip have a limited average particle size of 5 μm to 10 μm and the average crystal grain size D in mm and the number A of first particles per mm² in the aluminum alloy satisfy the condition √{square root over (D)}*A>1.8.

To be precise, if in the method for producing the sheet or strip, a heat treatment is performed by means of recrystallization annealing and subsequent accelerated cooling, then based on the composition and the resulting microstructure, this can ensure a sufficiently high number of dislocations in the sheet or strip. This prevents the formation of Lüders band dislocations even with complex geometries. According to the invention, this produces a sheet or strip composed of an aluminum alloy, preferably with an Al—Mg basis (or with Mg as one of the main alloying elements) in ssf quality or ffa quality, which due to its sufficient strength and formability can also excel when used, for example, for outer shell components in vehicle body construction.

The number of dislocations in the sheet or strip can be further increased if √{square root over (D)}*A is >2. More particularly, if √{square root over (D)}*A is >2.5, then the sheet or strip can satisfy comparatively high quality requirements without having to also fear the occurrence of stretcher strain marks such as type A Lüders bands on the surface of the formed sheet or strip, even in the case of comparatively complex geometries or unfavorable plastic deformations.

A sufficient number of dislocations in order to avoid stretcher strain marks in the formed sheet or strip can be achieved if the crystal structure has more than 200, more particularly more than 400, dislocations at each first particle. This can be achieved if the sheet or strip has been heat treated by heating and subsequent accelerated cooling in such a way that the crystal structure has more than 200, more particularly more than 400, dislocations at each first particle.

Preferably, the number A of first particles is ≥10 particles/mm², which can enable a sufficient distribution of the dislocations in the sheet or strip in order to avoid stretcher strain marks. This is more particularly the case if the number A of first particles is ≥25 particles/mm², preferably ≥35 particles/mm².

If the intermetallic phase has an Al—Mn basis, then it is possible to produce the dislocations in the aluminum alloy that enable stretcher strain marks to be avoided in a particularly reliable way. Preferably, the intermetallic phase is of the Al₁₃(Mn,Fe)₆ type or of the Al₁₆FeMn₃Si₂ type or of the Al₁₂Mn type or of the Al₆Mn type. These first particles of the primary phase are a particularly stable phase. It is also conceivable for the primary phase to constitute the intermetallic phase in order, through the subsequent heat treatment of the sheet or strip, to achieve a sufficient number of dislocations.

The method can achieve high strength and formability while avoiding orange peel and stretcher strain marks if the aluminum alloy has from 4.0 to 5.0 wt % magnesium (Mg) and/or from 0.2 to 0.5 wt % manganese (Mn).

Particularly high strength can be achieved if the aluminum alloy also has from 2.0 to 4.0 wt % zinc (Zn) (with an Al—Mg—Zn basis). Optionally, this aluminum alloy can also have up to 0.8 wt % copper (Cu).

The sheet or strip according to the invention can also be particularly well-suited for producing a molded part, more particularly a vehicle part, preferably a vehicle body part, by means of sheet-metal-forming. Preferably, the sheet or strip is used to produce a sheet bar in order to be able to perform a sheet-metal-forming process.

In general, it should be mentioned that the average crystal grain size and the average particle size are measured using the ASTM E112 linear intercept method.

Preferably, the aluminum alloy has an Al—Mg basis.

In addition, the sheet or strip can have an average crystal grain size D of ≤50 μm, ≤40 μm, or ≤30 μm.

In addition, the cooling rate (or cooling speed) can be <2.4° C./s, <2.3° C./s, <2.2° C./s, <2.1° C./s, <2.0° C./s, <1.9° C./s, <1.8° C./s, <1.7° C./s, <1.6° C./s, <1.5° C./s, <1.4° C./s, <1.3° C./s, <1.2° C./s, <1.1° C./s, <1.0° C./s, <0.9° C./s, <0.8° C./s, <0.7° C./s, or <0.6° C./s.

In general, it should be mentioned that the strip can be cut into a slit strip or cut into sheets or also sheet bars can be cut out from the sheet or strip in order to form these semi-finished products, for example by means of sheet-metal-forming. The forming can be a deep-drawing, roll profiling, etc.

In general, it should be mentioned that the aluminum alloy can, for example, be of the EN AW-5083 or EN AW-5086 or EN AW-5182 or EN AW-5454 or EN AW-5457 or EN AW-5754 type.

WAYS TO IMPLEMENT THE INVENTION

To demonstrate the achieved effects, cold-rolled semi-finished products, namely thin sheets composed of an aluminum alloy with an Al—Mg—Mn basis and thin sheets composed of and aluminum alloy with an Al—Mg—Zn—Mn basis were produced. The following aluminum alloys were used, which were composed of

TABLE 1 Different aluminum alloys Mg Mn Fe Si Zn Alloy wt % wt % wt % wt % wt % C1 4.57 0.41 0.19 0.12 — C2 4.71 0.41 0.23 0.12 — C3 4.88 0.41 0.18 0.12 — C4 4.74 0.44 0.24 0.12 — D1 4.70 0.45 0.23 0.13 3.5

The production of these thin sheets was carried out with the following process parameters:

TABLE 2 Overview of the production processes Casting Hot rolling Cold rolling Cooling Starting Degree of rolling rate temperature reduction after the Intermediate Heat Sheets Alloy [° C./s] [° C.] intermediate annealing annealing treatment A1 C1 0.7 530° C. 63% 385° C. 2 h 500° C. WQ A2 C2 1.8 530° C. 15% 385° C. 2 h 500° C. WQ A3 C3 1.8 530° C. 18% 385° C. 2 h 500° C. WQ A4.1 C4 1.8 530° C. 25% 385° C. 2 h 500° C. WQ A4.2 C4 1.8 530° C. 25% 385° C. 2 h 370° C. AC  A5 C4 1.8 530° C. 63% 385° C. 2 h 500° C. WQ A6.1 D1 1.8 530° C. 18% 385° C. 2 h 500° C. WQ A6.2 D1 1.8 530° C. 63% 385° C. 2 h 500° C. WQ WQ: Water quenching (as an example of an accelerated cooling) AC: Cooling in stationary air

These thin sheets were used to produce sheet bars—i.e. sheet blanks—which were formed, namely sheet-metal-formed, specifically deep-drawn, to produce a vehicle body part, namely a hood.

TABLE 3 Overview of the deep-drawn thin sheets Grain size D √D A √D. A Stretcher Sheets Alloy [μm] [mm^(0.5)] [mm⁻²] [mm^(−1.5)] strain marks A1 C1 15 0.12 44 5.4 No A2 C2 35 0.19 12 2.24 No A3 C3 29 0.17 14 2.38 No A4.1 C4 32 0.18 12 2.14 No A4.2 C4 32 0.18 12 2.14 Yes A5 C4 10 0.10 12 1.2 Yes A6.1 D1 28 0.17 14 2.34 No A6.2 D1 10 0.1 14 1.4 Yes

Exemplary Embodiment 1

An alloy of the AA5182 type (Al—Mg—Mn basis) with the chemical composition C1 was used to produce a thin sheet A1 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a comparatively reduced cooling rate (or cooling speed) and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 63% (from 3.25 mm to 1.2 mm) and the final heat treatment was carried out at 500° C. with subsequent water quenching. The average crystal grain size or final grain size of the thin sheet A1 was 15 μm (measured using the ASTM E112 linear intercept method) and in the primary intermetallic phase, there were 44 first particles per mm² having an average particle size of 5 μm to 10 μm (measured using the ASTM E112 linear intercept method). These primary particles were also embodied as comparatively coarse. In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 44, the condition 10≤degree of rolling reduction*cooling rate≤50 is satisfied.

With a √D*A value of 5.4, the criterion (√D*A>1.8) is satisfied. A tensile test did not show any Lüders bands on the surface of the thin sheet A1. The intermetallic phase according to the invention with the first particles was therefore able to provide a sufficient number of dislocations to prevent the occurrence of Lüders bands during the forming.

Exemplary Embodiment 2

An alloy of the AA5182 type with the chemical composition C2 was used to produce a thin sheet A2 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a cooling rate (or cooling speed) of 1.8° C./s and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 15% (from 1.41 mm to 1.2 mm) and the final heat treatment was carried out at 500° C. with subsequent water quenching. In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 27, the condition 10≤degree of rolling reduction*cooling rate≤50 is satisfied.

The average crystal grain size or final grain size of the thin sheet A1 after the heat treatment was 35 μm and in the primary intermetallic phase, there were 12 first particles per mm² having an average particle size of 5 μm to 10 μm. With a √D*A value of 2.24, the criterion (√D*A>1.8) is satisfied. A tensile test did not show any Lüders bands on the surface of the thin sheet A2. The intermetallic phase according to the invention with the first or primary particles was therefore able to provide a sufficient number of dislocations to prevent the occurrence of Lüders bands during the forming.

Exemplary Embodiment 3

An alloy of the AA5182 type with the chemical composition C3 was used to produce a thin sheet A3 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a cooling rate (or cooling speed) of 1.8° C./s and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 18% (from 1.46 mm to 1.2 mm) and the final heat treatment was carried out at 500° C. with subsequent water quenching. The average crystal grain size or final grain size was 29 μm and in the primary intermetallic phase, there were 14 first particles per mm² having an average particle size of 5 μm to 10 μm. In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 32, the condition 10≤degree of rolling reduction*cooling rate≤50 is satisfied.

With a √D*A value of 2.38, the criterion (√D*A>1.8) is satisfied. A tensile test did not show any Lüders bands on the surface of the thin sheet A3. The intermetallic phase according to the invention with the first or primary particles was therefore able to provide a sufficient number of dislocations to prevent the occurrence of Lüders bands during the forming.

Exemplary Embodiment 4

An alloy of the AA5182 type with the chemical composition C4 was used to produce two thin sheets A4.1 and A4.2 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a cooling rate (or cooling speed) of 1.8° C./s and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 25% from 1.60 mm to 1.2 mm). The final heat treatment of the thin sheet A4.1 was carried out at 500° C. with subsequent water quenching. By contrast, the final heat treatment of the thin sheet A4.2 was carried out at 370° C. with subsequent cooling in stationary air.

The average crystal grain size or final grain size of both of the thin sheets A4.1 and A4.2 was 32 μm and in their primary intermetallic phase, there were 12 first particles per mm² having an average particle size of 5 μm to 10 μm. With a √D*A value of 2.14, the criterion (√D*A>1.8) is satisfied by both thin sheets A4.1 and A4.2.

In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 45, the condition 10≤degree of rolling reduction*cooling rate≤50 is satisfied by both thin sheets A4.1 and A4.2.

By contrast with the thin sheet A4.1, the thin sheet A4.2 exhibits Lüders bands after the deep-drawing. In the thin sheet A4.2, despite having the same composition and microstructure, because of the slower cooling in stationary air, it was not possible for a sufficient number of dislocations in the structure to form in order to prevent the occurrence of Lüders bands. In other words, the accelerated water cooling of the thin sheet A4.1 resulted in the fact that the intermetallic phase with the first or primary particles was able to provide a sufficient number of dislocations to prevent the occurrence of Lüders bands during the forming.

Exemplary Embodiment 5

An alloy of the AA5182 type with the chemical composition C4 was used to produce a thin sheet A5 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a cooling rate (or cooling speed) of 1.8° C./s and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 63% (from 3.25 mm to 1.2 mm) and the final heat treatment was carried out at 500° C. with subsequent water quenching. The average crystal grain size or final grain size was 10 μm and in the primary intermetallic phase, there were 12 first particles per mm² having an average particle size of 5 μm to 10 μm.

With a √D*A value of 1.2, the criterion for freedom from Lüders bands (√D*A>1.8) is not satisfied. In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 113, the condition 10≤degree of rolling reduction*cooling rate≤50 is not satisfied. After the deep-drawing, Lüders bands were detected. The intermetallic phase with the first or primary particles was therefore not able to provide a sufficiently high number of dislocations to prevent the occurrence of Lüders bands during the forming.

Exemplary Embodiment 6.1

An alloy with an Al—Mg—Zn—Mn basis and the chemical composition D1 was used to produce a thin sheet A6.1 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a cooling rate (or cooling speed) of 1.8° C./s and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 18% (from 1.46 mm to 1.2 mm). The final heat treatment was carried out at 500° C. with subsequent water quenching. After the accelerated cooling, a stabilization was carried out at 100° C. for 3 h. The average crystal grain size or final grain size was 28 μm and in the primary intermetallic phase, there were 14 first particles per mm² having an average particle size of 5 μm to 10 μm. With a √D*A value of 2.34, the criterion (√D*A>1.8) is satisfied. In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 32, the condition 10≤degree of rolling reduction*cooling rate≤50 is satisfied.

A tensile test did not show any Lüders bands on the surface of the thin sheet A6.1. The intermetallic phase according to the invention with the first or primary particles was therefore able to provide a sufficient number of dislocations to prevent the occurrence of Lüders bands during the forming.

Exemplary Embodiment 6.2

An alloy with an Al—Mg—Zn—Mn basis and the chemical composition D1 was used to produce a thin sheet A6.2 with a sheet thickness of 1.2 mm. The rolling slab was solidified at a cooling rate (or cooling speed) of 1.8° C./s and the rolling steps in the hot rolling and cold rolling were carried out in accordance with the standard scheme. The last rolling reduction pass in the cold rolling amounted to 63% (from 3.25 mm to 1.2 mm) and the final heat treatment was carried out at 500° C. with subsequent water quenching. The average crystal grain size or final grain size was 10 μm and in the primary intermetallic phase, there were 14 first particles per mm² having an average particle size of 5 μm to 10 μm. With a √D*A value of 1.4, the criterion for freedom from Lüders bands (√D*A>1.8) is not satisfied. In addition, with the product of the cooling rate after the intermediate annealing and the degree of rolling reduction of 113, the condition 10≤degree of rolling reduction*cooling rate≤50 is not satisfied.

After the deep-drawing, Lüders bands were detected. The intermetallic phase with the first or primary particles was therefore not able to provide a sufficiently high number of dislocations to prevent the occurrence of Lüders bands during the forming.

All of the exemplary embodiments according to the invention, namely A1, A2, A3, A4.1, and A6.1 share the fact that their crystal structure has more than 200, more particularly more than 400, dislocations at each first particle.

In general, it should be noted that the German expression “insbesondere” can be translated into English as “more particularly.” A feature that is preceded by “more particularly” is to be considered an optional feature that can be omitted and therefore does not constitute a limitation, for example of the claims. The same applies to the German term “vorzugsweise,” which is translated into English as “preferably.” 

1. A method for producing a sheet or strip from an aluminum alloy, comprising: from 2.0 to 5.5 wt % magnesium (Mg), from 0.2 to 1.2 wt % manganese (Mn), optionally up to 0.45 wt % silicon (Si), up to 0.55 wt % iron (Fe), up to 0.35 wt % chromium (Cr), up to 0.2 wt % titanium (Ti), up to 0.2 wt % silver (Ag), up to 4.0 wt % zinc (Zn), up to 0.8 wt % copper (Cu), up to 0.8 wt % zirconium (Zr), up to 0.3 wt % niobium (Nb), up to 0.25 wt % tantalum (Ta), up to 0.05 wt % vanadium (V),

and a remainder comprising aluminum and inevitable production-related impurities, with up to at most 0.05 wt % of each and all together totaling at most 0.15 wt %, wherein the method comprises the following steps: casting a rolling slab; optional homogenization of the rolling slab; hot rolling the rolling slab into a hot-rolled sheet or strip; cold rolling the hot-rolled sheet or strip to a final thickness, optionally with an intermediate annealing of the sheet or strip, wherein the sheet or strip that has been cold-rolled to the final thickness has at least one intermetallic phase with first particles having an average particle size of 5 μm to 10 μm; and heat treatment of the sheet or strip that has been cold-rolled to the final thickness, including recrystallization annealing with subsequent accelerated cooling and optionally a stabilization of the sheet or strip that has undergone accelerated cooling, wherein the heat-treated sheet or strip has an average crystal grain size D of ≤60 μm and the average crystal grain size D in mm and a number A of first particles per mm² in the aluminum alloy satisfy the condition: √D*A>1.8.
 2. The method according to claim 1, wherein √D*A is >2.
 3. The method according to claim 1, wherein the recrystallization annealing takes place at 300° C. or more, and/or the accelerated cooling takes place at a cooling rate of at least 10 K/s, to below 180° C.
 4. The method according to claim 1, wherein the rolling slab is solidified by maintaining a cooling rate of <2.5° C./s.
 5. The method according to claim 1, wherein the optional homogenization takes place at 450° C. to 550° C. for at least 0.5 h, and/or the hot rolling takes place at 280° C. to 550° C., and/or the cold rolling to the final thickness, after the intermediate annealing, takes place with a degree of rolling reduction of 10% to 65%, and/or the optional intermediate annealing of the sheet or strip takes place at 300° C. to 500° C. and/or the optional stabilization takes place at 80° C. to 120° C. for at least 0.5 h.
 6. The method according to claim 5, wherein a product of the degree of rolling reduction in % after the intermediate annealing and the cooling rate in ° C./s satisfies the condition 10≤degree of rolling reduction*cooling rate
 50. 7. The method according to claim 1, wherein a primary intermetallic phase has an Al—Mn basis, and is of the Al13(Mn,Fe)6 type or of the A15FeMn3Si2 type or of the Al12Mn type or of the Al6Mn type.
 8. The method according to claim 1, wherein the aluminum alloy has from 4.0 to 5.0 wt % magnesium (Mg) and/or from 0.2 to 0.5 wt % manganese (Mn) and optionally from 2.0 to 4.0 wt % zinc (Zn).


9. A sheet or strip composed of an aluminum alloy comprising: from 2.0 to 5.5 wt % magnesium (Mg), from 0.2 to 1.2 wt % manganese (Mn), optionally up to 0.45 wt % silicon (Si), up to 0.55 wt % iron (Fe), up to 0.35 wt % chromium (Cr), up to 0.2 wt % titanium (Ti), up to 0.2 wt % silver (Ag), up to 4.0 wt % zinc (Zn), up to 0.8 wt % copper (Cu), up to 0.8 wt % zirconium (Zr), up to 0.3 wt % niobium (Nb), up to 0.25 wt % tantalum (Ta), up to 0.05 wt % vanadium (V),

and a remainder comprising aluminum and inevitable production-related impurities, with up to at most 0.05 wt % of each and all together totaling at most 0.15 wt %, wherein the sheet or strip has an average crystal grain size D of ≤60 μm and has at least one intermetallic phase with first particles having an average particle size of 5 μm to 10 μm, and wherein the average crystal grain size D in mm and a number A of first particles per mm² in the aluminum alloy satisfy the condition √D*A>1.8, wherein the sheet or strip has been subjected to a heat treatment, including recrystallization annealing with subsequent accelerated cooling and optionally a stabilization of the sheet or strip that has undergone accelerated cooling.
 10. The sheet or strip according to claim 9, wherein √D*A is >2.
 11. The sheet or strip according to claim 9, wherein the crystal structure has more than 200 dislocations at each first particle.
 12. The sheet or strip according to claim 9, wherein the number A of first particles in the aluminum alloy is ≥10 particles/mm.
 13. The sheet or strip according to claim 9, wherein a primary intermetallic phase has an Al—Mn basis, and is of the Al13(Mn,Fe)6 type or of the Al15FeMn3Si2 type or of the Al12Mn type or of the Al6Mn type.
 14. The sheet or strip according to claim 9, wherein the aluminum alloy has from 4.0 to 5.0 wt % magnesium (Mg) and/or from 0.2 to 0.5 wt % manganese (Mn) and optionally from 2.0 to 4.0 wt % zinc (Zn).


15. A molded part, more particularly a vehicle part, preferably a vehicle body part, composed of a sheet-metal-formed sheet or strip according to claim
 9. 16. A molded vehicle part composed of a sheet-metal-formed sheet or strip according to claim
 9. 17. A molded vehicle body part, composed of a sheet-metal-formed sheet or strip according to claim
 9. 